@sting8282bird
I have a 98 Lumina, and I suspect the head gasket has a leak. Wild temperature swings, even after thermostat and radiator cap replacement. Radiator and water pump seem to be working fine.
Did you replace both the front and back head gaskets? I suspect my back one is bad, and I don't know how tough a job that is. I replaced my lower intake manifold gaskets and front head gasket in 2003.
Or, how much would I expect to pay the man to do it?